Yosuga no Sora - In Solitude, Where We Are Least Alone. Manga by Sphere, Takashi Mikaze

The story is set in the distant mountain village of Okukozome-chou. Kasugano Haruka and his twin sister Sora have fond childhood memories of going to the village to visit their grandfather during their summer vacations. However, after losing their parents in an accident, the two must move in with their grandfather.

The twins get reacquainted with their old childhood friend Yorihime Nao, their new classmate Nakazato Ryouhei, the shrine maiden Amatsume Akira, and Akira's friend Migiwa Kazuha. All seems peaceful and familiar. However, beneath the surface is a secret promise that the twins made with each other long ago, the whereabouts of an important lost item, and the real reason they chose to come back here.